1. Field of Invention
The present invention relates to a method of forming a semiconductor device. More particularly, the present invention relates to a method of forming a composite silicon oxide layer over a semiconductor device.
2. Description of Related Art
In most semiconductor devices, doped silicate glass such as borophosphosilicate glass (BPSG) is generally used as an inter-layer dielectric (ILD). This is because doped silicate glass has a low annealing temperature, thereby possessing and ability to lower the thermal budget of fabrication. However, due to the high porosity of doped silicate glass, some of the dopants inside the silicate glass can easily diffuse into neighboring layers at a moderately high temperature. Thus, semiconductor devices under the silicate glass layer may be contaminated in the back-end process, leading to reliability problems.
To prevent the contamination of a semiconductor device by dopants inside silicate glass, an isolating layer is often formed between the semiconductor device and the silicate glass layer. In other words, before doped silicate glass is deposited to form the inter-layer dielectric (ILD), undoped ozone-TEOS oxide (USG), plasma-enhanced chemical vapor deposition (PECVD) oxide, or silicon-rich oxide (SRO approximated formula SiOx, x less than 2) is first deposited over the semiconductor device. Among USG, PECVD oxide and silicon-rich oxide, silicon-rich oxide is the best material for preventing device contamination.
However, using silicon-rich oxide as an isolation layer has its own problems too. In depositing silicon-rich oxide, hydrogen molecules are often produced. These hydrogen molecules are often retained inside the silicon-rich oxide after the reaction, leading to a possible contamination of nearby semiconductor devices. Consequently, hot carrier degradation, current leakage and resistivity change of semiconductor devices occur more frequently.
Accordingly, one object of the present invention is to provide a method of forming a composite silicon oxide layer over a semiconductor device so that hot carrier degradation, current leakage and resistivity change of the device are minimized.
To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides a method of forming a composite silicon oxide layer. The composite silicon oxide layer is formed between the semiconductor device and a doped silicate glass layer. This composite silicon oxide layer is composed of two silicon oxide layers, each having a different silicon-to-oxygen ratio. The silicon oxide layer that contacts the doped silicate glass layer is a silicon-rich oxide layer (SiOx, x less than 2), while the other silicon oxide layer that contacts the semiconductor device is a silicon dioxide (SiO2) layer.
In addition, the two silicon oxide layers having different silicon/oxygen ratios are formed in the same plasma deposition step. The plasma deposition is carried out using, for example, SiH4xe2x80x94N2O plasma. The method of controlling the silicon/oxygen ratios in each of the two silicon oxide layers includes changing the ratio of SiH4 to N2O in the SiH4xe2x80x94N2O plasma.
In this invention, since the silicon-rich oxide layer of the composite layer is in contact with the doped silicate glass layer, dopants inside the doped silicate glass layer are prevented from diffusing into and contaminating the semiconductor device underneath. On the other hand, since the silicon dioxide layer in the composite layer is attached to the semiconductor device, any residual hydrogen molecules in the silicon-rich oxide is prevented from crossing into the underlying semiconductor device. Therefore, resistivity of the semiconductor device can be maintained and leakage current from the semiconductor device can be minimized.
Furthermore, since the two silicon oxide layers are formed by plasma deposition reactions in the same plasma chemical vapor deposition chamber, there is no additional complication other than that of forming a single silicon-rich oxide layer.
In brief, the composite silicon oxide layer of this invention is capable of preventing dopants in the doped silicate glass layer as well as residual hydrogen molecules in the silicon-rich oxide layer from contaminating the semiconductor device. Hence, quality of the semiconductor can be maintained. Moreover, since the two silicon oxide layers are formed in the same reaction chamber, no additional steps need to be taken.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.